fix(cpp): handle void return type in generated test harness

[diegohh0411]

Summary

• generateCallCode(): no longer emits auto res = obj->method() for void-returning methods — calls the method without assignment instead
• generatePrintCode(): for void returns, prints the modified parameter via MetaData.Output.ParamIndex rather than res, or prints "null" when no output param is specified
• Matches the existing behavior in the Python, Go, and Rust generators

Fixes #405

Test plan

• go test ./lang/... -run "TestGenerateCallCode|TestGeneratePrintCode" -v — all 5 tests pass
• Built patched binary and verified problem 283 harness compiles and passes both test cases
• go test ./... — no regressions in full suite

🤖 Generated with Claude Code

[diegohh0411]

Hey @j178 I've been using your tool for a bit and I love it! I use C++ mainly for Leetcode & I came across a bug while solving Leetcode problem no. 283. If you do leetgo pick 238, solve it and then test it locally via leetgo test 238 -L, you get this error:

$ leetgo pick 283
● reading leetcode cookies browser=firefox domain=leetcode.com
● generated file=cpp/0283.move-zeroes/solution.cpp
● generated file=cpp/0283.move-zeroes/testcases.txt
● generated file=cpp/0283.move-zeroes/question.md
● none editor is used, skip opening files
$ leetgo test 283 -L
● reading leetcode cookies browser=firefox domain=leetcode.com
● running test locally question=move-zeroes
● building file=cpp/0283.move-zeroes/solution.cpp
[redacted]/cpp/0283.move-zeroes/solution.cpp: In function ‘int main()’:
[redacted]/cpp/0283.move-zeroes/solution.cpp:32:14: error: deduced type ‘void’ for ‘res’ is incomplete
   32 |         auto res = obj->moveZeroes(nums);
      |              ^~~
× failed to run test locally err="compilation failed: exit status 1"

This happens because the repository still didn't account for Solutions that return void because they should modify a param in-place. All other languages (Python, Rust, etc...) did account for this scenario, except C++. And this PR fixes it.

I used Claude Code to solve the issue fast, but I still made the testing myself manually. So I'm confident this is quality code.

[j178]

Thanks!